Generally, pyridine bases are a generic name of an alkyl-substituted pyridine including picolines and lutidines in addition to pyridine, which are important compounds as starting materials for preparing medicines or agricultural chemicals, solvents for reactions and, further solvents.
As processes for the preparation of the pyridine bases, there is a process in which an aliphatic aldehyde and ammonia are condensed under a gaseous phase, and it is publicly known that a crystalline aluminosilicate, so-called, zeolites are used as a catalyst in the process.
Furthermore, recently, it is also known that a zeolite manufactured by Mobil Oil Corporation ZSM family typified by ZSM-5/disclosed in, for example, U.S. Pat. No. 3,702,886 (Japanese Patent Publication Kokoku No. 10064/1971)! is particularly preferred for the process.
The zeolite is characterized by a Si/Al atomic ratio of not less than 10 and a constraint index of 1-12.
Also, in U.S. Pat. No. 4,861,894 (Japanese Patent Unexamined Publication Kokai No. 9974/1988), there is disclosed a process for the preparation of pyridine bases in which a crystalline silicate such as the ZSM family is employed as a catalyst in combination with copper, zinc, bismuth, chromium, molibudenum, tungsten, cobalt, nickel, ruthenium, rhodium, paradium, and iridium as co-catalytic metals.
However, the reaction yields are industrially less than satisfactory in spite of using the above-mentioned catalysts.
Specifically, Examples of the above-described U.S. Pat. No. 4,861,894 describe that the pyridine bases are synthesized using acetaldehyde and formaldehyde as starting materials to obtain pyridine yield of 47% and picoline yield of 17% (total yield of 61%). Furthermore, Japanese Patent Publication Kokoku No. 92368/1994 Japanese Patent Publication Kokai No. 181256/1987! and Japanese Patent Publication Kokoku No. 92369/1994 Japanese Patent Publication Kokai No. 139168/1988! describe processes in which there are employed zeolites in or on to which Tl, Co, and Pb compounds are carried.
However, the metals, particularly Tl and Pb are extremely toxic, and are highly dangerous from the viewpoint of environmental and human safety.
As a result of an intensive study in view of the above described situations, the present inventor has found that pyridine bases can be obtained with a surprisingly high yield by using a catalyst in which specified metal compounds or ions are carried in or on to zeolites by modifying, specifically, by doping, by mixing, or by ion-exchanging.